Previously believed to be the only way to replace the old and heavily polluting Mirant Potrero Power Plant, the plan has now fallen out of favor with the mayor and some supervisors because of environmental concerns.

Mayor Gavin Newsom now supports a separate proposal to retrofit three of the four current generators at the Mirant site instead of building new generators.

The California Independent System Operator, which oversees the state's energy grid, said the remaining generator could close when a project to build a power-transmission line to San Francisco from the East Bay is completed in 2010.